The removal of finely-divided solid particles from entraining gases is necessary in almost any system in which the gas is to be passed through a fluid dynamic device containing gas-deflecting walls, such as an expansion turbine at an expander, to prevent erosion damage to such systems. Additionally, if the entraining gas is ultimately to be discharged to the atmosphere, the removal of particulate matter is also important from an environmental conservation standpoint. Emission levels of below 50 mg/Nm3 are sometimes required because of these environmental restrictions.
Suitable separators for removal of finely divided solid particles from entraining gases are so-called third stage separators as for example described in Hydrocarbon Processing, January 1985, 51-54. Third stage separators remove to an acceptable level the fine particles still present in the gas stream leaving the fluid catalyzed cracker regenerator just up-stream of an expander turbine or flue-gas boiler. It has been found that third stage separators may also find application in other processes, wherein finely divided solid particles are to be separated from entraining gases. Examples of such processes are direct iron reduction processes, coal gasification processes, coal based power plants and calcining processes, such as aluminium calcining.
Third stage separators as for example described above comprise a plurality of parallel-arranged swirl tube separators. Swirl tube separators are cylindrical cyclones with an axial entry for a solids-gas mixture and are for example described in EP-B-360360, U.S. Pat. No. 4,863,500, U.S. Pat. No. 5,681,450, GB-A-1411136 and U.S. Pat. No. 3,541,766. In EP-B-360360 a swirl tube separator unit is described comprising of a gas outlet tube placed axially in the upper part of a vertically tubular housing, wherein an axial directed gas inlet means is provided by the annular space between the gas outlet tube and the vertically arranged tubular housing and wherein in said annular space swirl imparting means are arranged. In FIG. 2 of EP-B-360360 a swirl tube separator is shown having a vortex stabilizer comprising a pin placed on a base element. It is believed that the vortex will be kept in the center of the tubular housing because the lower end of the vortex will terminate on the pin.
It has been found that when such a cyclone separator is used on a feed containing a substantial amount of solids, such as is the case in a third stage separator, non-axial symmetric vortex development can occur. Such a non-preferred operation results in that the cyclone does not separate solids as optimally as required. Furthermore erosion of the inside of the tubular part of the cyclone has been observed.
U.S. Pat. No. 4,810,264 discloses a cyclone separator having a tangentially arranged inlet for gas and solids. Below the gas outlet conduit an adjustable vortex stabiliser in the form of a flat plate or a disc is positioned. On this stabiliser a vortex finder is positioned. According to this publication the distance between vortex stabiliser and the gas outlet opening is frequently adjusted such to vary the particulate content of the gas exiting through the gas-outlet opening.
U.S. Pat. No. 1,753,502 discloses a pneumatic dust collector. The dust collector consists of a cyclone having a tangentially arranged gas-solids inlet opening. From the gas outlet conduit a stem extends to a disk positioned below the gas outlet opening.
EP-A-052042 discloses a swirl tube separator provided with anti-erosion means fixed on the interior wall of the housing of the separators.
U.S. Pat. No. 4,795,561 discloses a cyclone separator provided with a tangential arranged gas-solids inlet, a cylindrical housing with a closed bottom and a valve at the bottom of the cylindrical housing. The valve is fixed to a pin. The opposite end of this pin is located in the gas outlet conduit present at the upper end of the cylindrical housing. The pin thus mechanically positions the valve and the movement of the valve within the separator.
U.S. Pat. No. 4,072,481 discloses a device for separating a gas from a mixture of a liquid, solids and gas. The inlet for the mixture is tangential. A so-called stand provided with a plate at its upper end is present at some distance below the outlet for the gaseous phase.
U.S. Pat. No. 4,795,561 discloses a cyclomic separator for use in fluid flow systems that comprises a housing containing an inlet, a moveable shroud, and at least one variable area outlet.
The object of the present invention is to provide a swirl tube separator having improved separation efficiency and which has a lesser tendency to operate with a non-symmetric vortex.